Application Architecture

The J2EE Application model, as shown in the following figure, is very
flexible; allowing the application architect to split application logic functionally
into many tiers. The presentation layer is typically implemented using servlets
and JSP technology and executes in the web container.

Figure 1–1 J2EE Application Model

Moderately complex enterprise applications can be developed entirely
using servlets and JSP technology. More complex business applications often
use Enterprise JavaBeans (EJB) components. The Application Server integrates
the web and EJB containers in a single process. Local access to EJB components
from servlets is very efficient. However, some application deployments may
require EJB components to execute in a separate process; and be accessible
from standalone client applications as well as servlets. Based on the application
architecture, the server administrator can employ the Application Server in
multiple tiers, or simply host both the presentation and business logic on
a single tier.

It is important to understand the application architecture before designing
a new Application Server deployment, and when deploying a new business application
to an existing application server deployment.